Cor pulmonale develops a disease condition in which an increase in the pulmonary vascular resistance due to an organic or functional abnormality of the lungs causes right ventricular pressure overload, resulting in right ventricular hypertrophy and further right ventricular insufficiency. Cor pulmonale is caused by various diseases, the critical mechanism of ventricular hypertrophy is not clarified, and no curative means is established. Although cor pulmonale is treated by causal disease treatment (including oxygen therapy) or using a diuretic and cardiotonic, an effective curative means has not yet been established.
On the other hand, prostaglandin I.sub.2 (PGI.sub.2, prostacyclin) which is known as a substance having the strong actions of inhibiting platelet aggregation and dilating the peripheral vessel (refer to "Nature" Vol. 268, p688, 1976) has an unstable exoenol structure, and thus PGI.sub.2 is very unstable in a neutral aqueous solution and is converted into 6-oxo PGF.sub.1 .alpha. which has substantially no physiological activity. This instability of PGI.sub.2 brings about an important problem in utilizing this compound as a medicine. The PGI.sub.2 is also unstable in vivo and thus has the fault that its physiological action has no persistency. Japanese Examined Patent Publication No. 1-53672 discloses, as compounds in which the faults of PGI.sub.2 are significantly improved, PGI.sub.2 derivatives having a skeleton in which the structure of the exoenol ether portion, that is a characteristic structure of PGI.sub.2, is converted into an inter-m-phenylene type. However, this publication does not suggest the curative effects of those derivatives on cor pulmonale, and it has not yet been known that the PGI.sub.2 derivatives have the curative effects on cor pulmonale.
As a result of extensive research for developing an agent for treating cor pulmonale having excellent efficacy and practicability, the inventors found that the compounds used in the present invention have the significant effect of ameliorating the condition of cor pulmonale, resulting in the achievement of the present invention.